1. Field of the Invention
The present invention relates to novel fluorochemical surfactants and in particular, it relates to the use of novel fluorochemical surfactants in photothermographic and thermographic elements. The use of fluorochemical surfactants in coating compositions reduces disuniformities, such as mottle, in photothermographic elements.
2. Background of the Art
Silver halide-containing photothermographic imaging materials (i.e., heat-developable photographic materials) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive material that generates elemental silver when irradiated; (2) a non-photosensitive, reducible silver source; (3) a reducing agent for the non-photosensitive, reducible silver source; and (4) a binder. The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the non-photosensitive, reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and the photo-sensitive, photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive, photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. The preferred non-photosensitive, reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
One method of attempting to increase the maximum image density in black-and-white photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating toning agents into the emulsion. Toning agents improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. For example, color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this manner, a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. Upon heating, typically in the range of about 60.degree.-225.degree. C., a reaction occurs only in the heated areas, resulting in the formation of an image.
Thermographic elements whose image-forming layers are based on silver salts of long chain fatty acids, such as silver behenate, are also known. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (1) a thermally sensitive reducible silver source; (2) a reducing agent for the thermally sensitive reducible silver source; and (3) a binder. Upon heating, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, leuco dyes, and the like, whereby an image comprised of elemental silver is formed.
Photothermographic and thermographic constructions are usually prepared by coating from solution and removing most of the coating solvent by drying. One common problem that exists with coating photothermographic systems is the formation of coating defects. Many of the defects and problems that occur in the final product can be attributed to phenomena that occur in the coating and drying procedures. Among the problems that are known to occur during drying of polymeric film layers after coating is unevenness in the distribution of solid materials within the layer. Examples of specific types of coating defects encountered are "orange peel", "mottling", and "fisheyes". "Orange peel" is a fairly regular grainy surface that occurs on a dried, coated film, usually because of the action of the solvent on the materials in the coating composition. "Mottling" often occurs because of an unevenness in the removal of the solvent from the coating composition. Fisheyes are another type of coating problem, usually resulting from a separation of components during drying. There are pockets of different ingredients within the drying solution, and these pockets dry out into uneven coating anomalies.
Surfactants have often been used to correct these types of problems, along with changes in the solvents of the coating compositions. In some cases, surfactants do not correct the problem, and in other cases the surfactants create other problems even when they cure the first problem. It is sometimes necessary to investigate a large number of commercially available surfactants before finding one that is appropriate for a particular type of system, even if that commercial product is touted for use in correcting a particular type of defect.
For a surfactant to be useful in an imaging element is must have several properties. It must be soluble in the coating solution or emulsion. If it were not, then other defects such as fish-eyes and streaks may occur in the dried coating The surfactant must not stabilize foams or air bubbles within the coating solution or emulsion as these cause streaks in the dried coating. These defects are readily visible and are unacceptable in a final element. Additionally, the surfactant cannot significantly alter the sensitometric properties of the imaging element such as speed, contrast, minimum density, and maximum density.
Fluorochemical surfactants are useful in coating applications to reduce mottle. When a coating solution is dried at high speeds in an industrial oven, the resulting film often contains a mottle pattern. This mottle pattern is often the result of surface tension gradients created by non-uniform drying conditions. When an appropriate fluorochemical surfactant is added to the coating solution, the surfactant holds the surface tension at a lower, but constant value. This results in a uniform film, free from mottle. Fluorochemical surfactants are used because organic solvents, such as 2-butanone (also known as methyl ethyl ketone or MEK), already have such low surface energies (24.9 dynecm) that hydrocarbon surfactants are ineffective.
Allowed copending U.S. patent application Ser. No. 07/966,458 describes the use of fluorochemical surfactants to reduce coating disuniformities such as mottle, fisheye, and foaming in positive-acting or negative-acting resist systems such as printing plates and other non-resist imageable polymerizable systems. These polymers are comprise a fluorochemical acrylate, a short-chain-alkyl acrylate, and a polar monomer. Use of these materials in photothermographic or thermographic elements is not discussed.
U.S. Pat. Nos. 4,764,450 and 4,853,314 describe the use of particular changes in solvent systems to improve surface defects in positive-acting photoresist imaging systems.
U.S. Pat. No. 4,557,837 describes fluorochemicals useful in the preparation of foamable compositions such as those used in the cleanup of gas wells. Polymers described include copolymers of fluorochemical monomers and hydroxyethylacrylate, and copolymers of fluorochemical monomers, acrylic acid, and short chain acrylates.
JP 01-223,168 describes fluorinated terpolymers that are useful additives to varnish formulations. They improve the stain resistance of the varnish.
JP 57-040579 describes fluorinated terpolymers which are useful as release coatings for adhesive tapes.
U.S. Pat. No. 3,950,298 describes thermoplastic fluorinated terpolymers that are useful, non-foaming additives to coating solutions for polymeric materials such as carpets and fibers. The coating compositions provide oleophobicity to the surfaces that are coated.
U.S. Pat. No. 4,051,278 describes a process where a foraminous shield (such as a screen or perforated plate) is used to protect the coated web from the impingement air used for dying. Both solvent-rich and solvent-poor air can flow through the shield. Air velocity and turbulence are reduced by the porous shield. Although this method is claimed to reduce the degree of mottle, the amount and presence of mottle was still influenced by increased flow rate of the impingement air.
U.S. Pat. No. 4,999,927 describes an oven system for which the air flow boundary layer along the web remains laminar. This is accomplished by accelerating the air through the drying chamber.
U.S. Pat. No. 4,894,927 describes a technique for reducing mottle by combining an inert gas system with a small drying chamber. Using this method, the air flow remains laminar over the web.
U.S. Pat. No. 3,573,916 describes the use of sulfo-substituted cyanine dyes to reduce mottle in color-bearing silver halide emulsions which have been coated on electron bombarded hydrophobic surfaces.